Electrochemical machining

ABSTRACT

An electrochemical machining electrolyte which forms a specific electrochemical erosion inhibiting film, which film is susceptible to removal by the application of high current densities and potentials thereto. The principal ingredient of the preferred electrolyte for forming this specific film is at least one salt selected from the group consisting of sodium chlorate, potassium chlorate, sodium perchlorate and potassium perchlorate. pH modifiers such as sodium hydroxide, sodium carbonate, sodium borate and the like may be added. pHs between about 6.7 and 11 are the most effective.

United States Patent La Boda [451 June 13, 1972 ELECTROCHEMICALMACHINING Primary Examiner-J Mack Assistant Examiner-Neil A. Kaplan [72]Inventor Mncheu La Bods East Damn Mich Attorney-R. J. Wallace, WilliamS. Pettigrew and Lawrence [73] Assignee: General Motors Corporation,Detroit, 5 Plant Mich.

[22] Filed: May 21, 1970 ABSTRACT [211 App]. No.: 37,494 Anelectrochemical machining electrolyte which forms a specificelectrochemical erosion inhibiting film, which film is Related ApphcammDam susceptible to removal by the application of high current den- 3Continuation f 664,770, Aug 31 1967 sities and potentials thereto. Theprincipal ingredient of the hi h i a continuatiomimpan f s N 44 3 9preferred electrolyte for forming this specific film is at least April7, 1965. one salt selected from the group consisting of sodium chlorate.potassium chlorate, sodium perchlorate and potassium [52] U.S. Cl ..204/143 M perchlorate. pH modifiers such as sodium hydroxide, sodium [51]Int. Cl l ..B23p 1/00 carbonate. sodium borate and the like may beadded. pHs [58] Field of Search ....204/ 143 M between about 6.7 and 1 lare the most effective.

[56] References Cited UNITED STATES PATENTS 4 Claims, 5 Drawing Figures2,805,197 9/1957 Thibault et al ..204/l43 IOOO- NQCL RT E O U u 5 100- ia NQC LO 3 5 Q |o 2 [LI m 01 I I I i. I

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ANODE POTENTIAL PKTENTEBJM 13 1972 SHEET 2 BF 2 25p IN.

INVENTOR.

AFTER f (RMS) BEFORE (RMS) 133g m.

BY M fie/1e! 1 Za 300 0 ATTORNEY ELECTROCHEMICAL MACHINING This is acontinuation of application Ser. No. 664,770, filed Aug. 31, 1967, inthe name of Mitchell A. LaBoda, which was in turn a continuation-in-partof application Ser. No. 446,389, filed on Apr. 7, 1965, whichapplications were assigned to the assignee of the instant applicationand are now abandoned.

This invention relates to electrochemical machining and moreparticularly to an improved electrochemical machining electrolytesolution and to the process for using it.

Numerous advances have recently been made in electrochemical machiningequipment. However, despite these mechanical and electrical apparatusimprovements, little advance has been made in the development ofelectrolyte solutions for electrochemical machining. Aqueous sodiumchloride solutions are still most commonly used. Aqueous solutions ofother inorganic salts, such as nitrates and nitrites, have also beenused. However, these other solutions do not offer especially significantadvantages over the known and accepted aqueous sodium chloridesolutions. Hence, aqueous sodium chloride electrolyte solutions arestill predominantly preferred. However, all of these electrolytesolutions suffer from the disadvantage of producing uncontrolled anodicdissolution of the workpiece in unwanted areas. Hence, they produceovercutting, tapering of holes, rounded edges on flat surfaces and thelike. Anodic dissolution can occur with such electrolyte solutions evenin areas which are fairly well removed from the cathode. Wild cutting,that is, cutting in low current density areas that are continuallybathed in the electrolyte but substantially removed from the cathodegenerally occurs. In addition, these electrolyte solutions frequentlyproduce splash cutting, that is, superficial anodic dissolution in areaswhich are only intermittently wetted by splashes or sprays ofelectrolyte. Further, these electrolyte solutions do not produce highquality surface finishes, particularly at higher metal removal rates.

These inherent electrolyte solution problems are well known and sosevere that many mechanical and electrical innovations have been made inthe attempt to overcome them. However, the mechanical and electricalinnovations made so far only solve a few of these problems. None solvesthem all. I have found a chemical means to solve them. I have found anew electrolyte solution which actually eliminates these problems inmost cases and at least substantially reduces them in all cases. I havefound an electrolyte solution which produces such controlled anodicdissolution that it can produce a significantly improved surface finisheven at appreciably higher than normal metal removal rates.

I have discovered that electrochemical machining processes can beimproved by utilizing certain electrolytes which form uniqueelectrochemical erosion inhibiting films which are susceptible to localremoval by the application of high potentials and correspondingly highcurrent densities to the localized regions where cutting is desired andwhich mask the workpiece in the lower current density areas while at thesame time permitting high metal removal rates, excellent dimensionalcontrol and near perfect current efficiencies. The films are selectivelydestroyed by the concentration of high current densities in the areasthat are to be machined. Removal of the film in those areas permitscontinuous cutting thereat. These areas are cut at a surprisingly highrate when one considers that a polarizing film is first being formed andthen apparently destroyed. The surrounding areas which are subjected tolower current densities and stray currents retain the film and hence arenot machined. Hence, electrolytes which perform in accordance with myfindings permit rapid cutting rates in the high current density areasand virtually no cutting in the lower current density areas. Excellentdimensional control results.

An especially significant electrochemical machining electrolytesolution, which I have found, which looms far superior to any otherheretofore known or appreciated electrolyte and which fulfills all theheretofore recited qualifications, is an aqueous solution of at leastone salt selected from the group consisting of sodium chlorate,potassium chlorate, sodium perchlorate and potassium perchlorate. Inaddition to providing high cutting rates, excellent dimensional controland near perfect current efficiencies heretofore referred to, thechlorate and perchlorate electrolytes produce unexpectedly superiorsurface finishes. The electrolytes are useful for electrochemicalmachining with or without a mechanical assist. The former is morefrequently referred to as electrolytic grinding, while the latter ismore frequently simply referred to by its generic name electrochemicalmachining.

It is therefore an object of this invention to provide an improvedelectrochemical machining electrolyte solution and a process for usingit which produces extremely rapid but highly controlled and efficientmetal dissolution to permit faster electrochemical machining with closertolerances, and improved surfaces of the machine parts.

FIG. 1 is a semi-logarithmic plot of the potentiostatic curve reflectingthe comparison between sodium chloride and sodium chlorate.

FIG. 2 is a diagram depicting a two-dimensional projection of athree-dimensional surface reflecting the relationship between the metalremoval rate, electrolyte temperature, and electrolyte concentration.

FIG. 3 is a diagram depicting a two-dimensional projection of athree-dimensional surface reflecting the relationship between the metalremoval rate, the voltage and the electrolyte pump pressure.

FIG. 4 depicts the dimensional control capabilities of NaClO inelectrolytic grinding applications, and

FIG. 5 depicts the surface finish obtained after electrochemicallymachining a screw machined, heat treated sample with NaClQ With respectto my preferred electrolyte (alkali metal chlorates and perchlorates) Ifind that, for most applications, the addition of any of the other morecommonly used inorganic electrolyte salts to my electrolyte solutiondecreases its effectiveness and increases the tendency for the bath toproduce wild cutting or splash cutting. However, my new electrolyte doeshave a limited tolerance for certain pH controlling additives, notablysodium hydroxide. While I prefer to use sodium hydroxide for raising thepH, for certain applications, other pH controllers may be used such assodium carbonate, potassium hydroxide, sodium borate and sodiummetaborate. For most applications, and especially for the machining offerrous metals, compounding a bath containing any of the more commonlyused electrolyte salts tends to offset the dimensional control advantageof my invention. Accordingly, save for small quantities of pHcontrollers, in particular applications, my preferred solutions aresubstantially free of electrolytes other than sodium chlorate, potassiumchlorate, sodium perchlorate and potassium perchlorate. While the sodiumand potassium perchlorate salts are significantly improved over sodiumchloride as electrolytes, the sodium and potassium chlorate salts areeven better. Hence, I generally prefer to use these latter two salts.However, all four of these salts produce materially significantreductions in over cutting, wild cutting and splash cutting as well asimprovement in surface finish over any other inorganic electrolyte orsalt mixture that is known.

Even small but effective amounts of my selected salts can be used toobtain the advantages of this invention. However, when more highlyconcentrated electrolytes are used, higher current densities can beemployed in the process. With higher current densities, higher metalremoval rates can be obtained. Consequently, I prefer to use moreconcentrated solutions and generally feel that it is impractical toemploy solutions having concentrations less than about grams per liter(g/l). Lower concentrations (e. g., 40 g/l) can e used but at thesacrifice of solution conductivity. For production applications and easeof control, I believe that a salt concentration of approximately 350 g/lwould be generally preferred. Tests have shown, for example, that themetal removal rate increases quite sharply as the concentration ofsodium chlorate is increased up to about 600 g/l. As the concentrationis increased even further, however, the cutting rate still increases,but at a lesser rate. Hence, in the absence of a particular need for themaximum cutting rate, the economies dictate the use of a bath having nomore than about 600 g/l of sodium chlorate with about 350 g/l beingpreferred. However, as indicated, any salt concentration will beeffective in producing the intended results of this invention even up tosaturation. FIG. 2 shows the effects of sodium chlorate concentration onthe cutting rate. As indicated in FIG. 2, salt concentrations (2 axis)of approximately 1,000 g/l of sodium chlorate have been found to behighly effective, though not necessary, for most applications. In somecases, e.g., higher bath temperature operations, concentrations ofapproximately 1,500 g/l may be useful. However, it appears thatconcentrations of sodium perchlorate in excess of approximately 700 g/ldo not increase benefits to the same extent as the chlorate salt. Thus,it is generally not preferred to use a sodium perchlorate concentrationover about 700 g/l. The potassium salts, particularly potassiumchlorate, provide the control benefits of the invention but are lesssoluble and may unduly limit the cutting rate. Hence, the sodium saltsare generally preferred.

Incidentally, as is known in the art, some ferrous alloys (e.g., steel)have to be activated in mineral acids, acetic acid, or the like beforethey can be electrochemically treated. This invention does not eliminateactivation. However, it can simplify activation, if one desires.Activation of ferrous metals can be achieved in a plurality of ways. Forexample, the parts can be immersed in concentrated hydrochloric acid forapproximately 1 minute immediately prior to electrochemical machining.The part is then rinsed and machined. On the other hand, if a smallamount of activating acid is added directly to my electrolyte solution,one can activate the steel right in the machining electrolyte. Itappears that my electrolyte solution can tolerate up to about 3 percentby volume of concentrated activating acid before its basic, new anddifferent properties are drastically affected. This amount of acid isgenerally much more than enough to produce electrolytic activation.Generally about O.5l.0 percent by volume concentrated acid issufficient. However, even this small amount of activating acid actuallydoes tend to reduce the benefits realizable with my electrolytesolution. Hence, I generally prefer not to include any activating acidsat all in the electrolyte solution and to employ the usual separatetreatment for activation especially in those instances where pH controlis desired. In this way the integrity of my electrolyte solution ispreserved and the full benefits of the invention can be realized.

The following serves as a specific example of my invention. A ferrousmetal work piece of 0.7 percent carbon, 0.35 percent manganese, 0.25percent silicon, 1.0 percent chromium, 1.75 percent nickel and thebalance iron, having a surface area to be machined of about 1.3 squareinches, is cleaned in the usual manner before electrochemicallymachining. If its surface has been allowed to become oxidizedsufficiently to passivate it, it is preferably initially activated byimmersion in concentrated hydrochloric acid for about 1 minuteimmediately before it is machined. After activation, it is then rinsedand connected as the anode in a suitable electrochemical machiningapparatus, and spaced about 0.0l inch from an appropriately shapedcopper cathode. An aqueous, room temperature electrolyte solutionconsisting of about 500 g/l of sodium chlorate in water is then applied,to flood the interface between the anode and cathode. A sufficientpositive potential is then applied to the work piece to induce an anodiccurrent density of about 250 amperes per square inch on it. This currentdensity is continued until sufficient stock removal is obtained. Theinterface between the anode and cathode is con tinuously flushed withelectrolyte during the mixing means to provide best results. In suchcircumstances, about 0001-0002 inches per second is easily removed fromthe surface area machined, and surface finishes as low as 2 microinchesare obtained, with no wild cutting, splash cutting or the like. I havefound that it is generally necessary to use an anodic current density ofat least about 200 amperes per square inch in order to get sufficientlyrapid metal removal rates. The maximum permissible anodic currentdensity useful in the process is determined by the point at which arcingand/or sparking occurs. This point depends on a plurality of factors,including electrode gap spacing, salt concentration in the electrolyte,etc., as is well understood in the art. Anodic current densities ofabout 250-500 amperes per square inch are presently generally preferredfor commercial application because they not only provide sufficientlyfast metal removal but also produce an extremely good surface finish.However, much higher current densities can be used. In fact, I havefound that significantly higher current densities can be used with mypreferred electrolyte than with sodium chloride solutions. Hence, I canobtain significantly higher metal removal rates with my electrolyte thanwith sodium chloride solutions and the like. In this connection see HG.2 wherein some comparative data re sodium chloride is shown by brokenlines superimposed onto the sodium chlorate curves. In another series oftests, FIG. 3 illustrates the relative effects of current densities(shown as applied voltage) and flow rates (shown as pump pressure) onthe metal removal rate. In FIG. 3, the X-axis is plotted in terms of thepotential applied to the system (volts) rather than as currentdensities. This was done because the rapid area changes of the anodeduring machining made current density readings an impractical way ofdemonstrating the effects of this variable. The Z-axis is plotted interms of pump pressure rather than in termsof electrolyte velocity. Thiswas done because the electrode spacing increased during machining andhence the velocity actually varied continuously from beginning to end ofthe test run. in FIG. 3, it is seen that for any given current density(expressed as impressed potential) the increase of electrolyte flow rate(expressed as pump pressure) has little further influence on metalremoval rates after a certain minimum value necessary to sustainelectrochemical machining is attained. Clearly electrolyte flow is notas crucial using my electrolyte as it has been with other electrolytes.However, some increase in cutting rate is noted at the higher pressures.

Increasing the current density increases the metal removal rate. Inanother series of experiments conducted on SAE 5l60-H steel hardened toRockwell 60C, wherein the electrolyte flow rate was held constant, itwas determined that some additional mechanical action was necessary formachining below about 15 amps/cm amps/in In this same series ofexperiments the current density was gradually increased. For currentdensities in excess of about 27-31 amps/cm (-200 amps/in no additionalmechanical action was required to completely remove the film. In theintermediate current density range, 15-27 amps/cm (l00175 amps/in therewas substantial, though nonetheless incomplete, destruction of the filmby the electrochemical action. It was apparent therefore that when thecurrent density was increased to a high enough value, the film whichprevented electrochemical machining was destroyed and machiningproceeded. At the lower current densities, however, some additionalmechanical action was necessary. In this particular series of tests theinhibiting film was identified. Vacuum fusion and electron probeanalysis indicated a high percentage of oxygen on the surface of thework piece. Reflection electron diffraction studies identified a surfacefilm of either alpha or gamma Fe O Sodium chloride did not produce sucha film. Other compounds appear to be able to produce chemically similarfilms but the properties of these other films are not comparable to thatformed by the chlorates in that the dimensional control was not as good,poorer surfaces were produced, cutting rates were low and anode currentefficiency was only about 70 percent. The reasons for these differencesare not yet clearly known.

Still another series of experiments verified the previous findings.These latter experiments were conducted using an electrolytic grindingapparatus. Using this apparatus it was noted that, with sodium chlorate,cutting took place at lower current densities than was possible withoutthe mechanical action normally incident to electrolytic grindingtechniques. Hence, without this mechanical action, higher potentials andcurrent densities would be necessary to provide the same amount ofelectrochemical cutting. This observation substantiates the previousfindings that there appears to be a minimum current density below whichcutting will not occur without a mechanical assist and that above thisminimum no mechanical assist is required. Some tests have shown thisminimum to be at about 100 amps/in The electrolyte solution can be usedat any temperature up to its boiling point. The solubility of the sodiumchlorate, potassium chlorate, sodium perchlorate and potassiumperchlorate is very significantly increased with temperature. I havefound that increases in temperature from 50-l40 F. showed markedincreases in the metal removal rate. However, above 140 F. the metalremoval rate increased only slightly. The temperature variable shown onthe X-axis of FIG. 2 reflects this relationship. Hence, for fastercutting it is frequently desirable to heat the electrolyte solutionabove room temperature, preferably to about l40-l60 F. There doesntappear to be any significant benefits to be obtained by operating aboveabout 160 F. On the other hand, higher temperature operation requiresspecial equipment to establish and maintain a predetermined electrolytesolution temperature. Thus, for some purposes, such as commercialproduction operations, it is often desirable to use my electrolytesolution at room temperatures and forego the benefits of highertemperature operation.

It is interesting to note the effects of only slight variations in thepH of my unique electrolyte on different materials. A stock solution ofmy electrolyte (e.g. 35 percent by weight solution of sodium chlorate inwater) may be used to machine a variety of work pieces. Such a solutionhas a normal pH of approximately 6.7. However, I have found that byvarying the pH, ever so slightly, I can actually tailor the electrolytefor a given work piece and thereby attain better and in some cases farsuperior results than is possible with the stock solution. For example,using sodium hydroxide as the pH controller, I have found that suchsteel as SAE 5160-H, 8620, and pearlitic malleable cast iron machinebest at pHs between about 7-8. Other steels such as SAE J525 are bestmachined at pHs of about 10.4. The stainless steels generally requirehigher pHs such as about 8-12 for 410 stainless steel and 10-13 for 304stainless steel. Generally speaking, pHs of about 8.5 appear to be aboutthe optimum for most work. Approximately 10 to 20 percent increasedmetal removal rates were experienced on occasion when stock solutionswere so adjusted. In certain instances, increasing the pH up to 11showed improved results over the stock solution. Increasing the pHbeyond about 1 1, however, generally decreased metal removal rates formost steels, except the stainless steels. Though, as indicated, theborates are effective as pH controllers, they havea significantdisadvantage. The borates tend to crystalize out of solution underelectrochemical machining conditions. As a result, pumps, hoses, and gapspaces can become clogged which contributes to the breakdown ofequipment. Each of the other pH modifiers has its own peculiardisadvantage (e.g., degree of ionization) when compared to the benefitsobtainable from the use of sodium hydroxide. Hence, while many pHmodifiers may be employed, sodium hydroxide is still preferred.

While I do not intend to be bound by any particular theory concerning myinvention, it appears that the chlorates and perchlorates form uniquedestructible electrochemical erosion inhibiting films over the surfaceto be machined. The exact nature of, and the reason why, these uniquefilms work and others do not is not clearly understood. Some other saltsdo form films. These other films do inhibit electrochemical erosion andin some instances are even destructible under high current densityconditions. Even so, none of these other salts have produced films whichpermit the high metal removal rates. superb dimensional control,excellent surface finishes and near perfect current efficienciesexperienced with the use of the chlorates and perchlorates.Potentiostatic measurements, such as shown in FIG. 1, give a clue as tothe polarization characteristics that an electrolyte must have if it isto perform satisfactorily as an electrochemical erosion inhibitingfilm-forming electrolyte constituent. However, polarization informationalone, such as reflected by FIG. 1, does not tell the whole story, noris it enough to determine which compounds will be suitable for thisimproved variety of electrochemical machining. any event, when usingfilm forming electrolytes, the film must be removed from the region tobe machined before machining can commence or continue. In electrolyticgrinding techniques this film is removed mechanically as by the use ofabrading cathodes. Where true electrochemical machining is desired, thefilm is broken electrochemically by raising the potential and currentdensity to a sufficiently high value. Below this value no machining willtake place without the aid of a mechanical assist. Hence, controlledcutting can be effected by selectively destroying the film in thoseregions where cutting is desired and this is accomplished by selectivelyapplying high potentials and current densities to the areas to bemachined. The surrounding areas which are subjected to lower potentials,current densities and stray currents retain the film and hence are notmachined. When operating at current densities below the criticalelectrochemical value required to destroy the film, mechanical meansmust be employed to break the film before machining can commence. Insome cases the only mechanical assist required is that which resultsfrom the shearing forces of increased electrolyte flow rates. Generally,lower current density operations require somewhat higher electrolyteflow rates than do the higher current density operations to effect thesame cutting rates. Therefore, when using my improved electrolyte eitherthe flow rate or the current densities or both can be varied to effect aparticular machining result. Increasing either will increase the amountof cutting with the potential and current density being the moresignificant variables. In certain cases pH is also a significantvariable. In the remote areas where the current density is below thecutting current density and the mechanical action is insufficient toremove the film, no metal dissolution takes place and wild cutting" iseliminated.

I have found that chromium, molybdenum, copper, zinc, stainless steeland a variety of other ferrous alloys, particularly through-hardenedsteels and tool steels, can be effectively machined with my basicelectrolyte or modified version thereof. It appears that thiselectrolyte is useful in electrochemical machining of many, though notall, other metals also. In its basic form, for example, it is notparticularly effective for machining aluminum, as there is nosubstantial reduction of the wild cutting" as compared to otherelectrolytes. Although some surface improvement is effected on aluminum,even that isnt as good as produced on other metals. On the other hand,it is especially useful from a commercial standpoint toelectrochemically machine all ferrous alloys (i.e., those alloyscontaining more than 50 percent iron), particularly ferrous alloys whichare otherwise difficult to machine electrochemically, such as pearliticmalleable cast iron and carbide steels. My electrolyte solution exhibitsimproved machining results on these metals, whether they are beingeroded by the electrolytic grinding technique or by simple anodicdissolution without any mechanical assist.

The most unexpected and highly desirable benefit that I have found withmy electrolyte solution is that it produces surface finishes in nearlyevery case that are substantially improved over those produced withsodium chloride electrolyte solutions, even at the high metal removalrates. FIG. 5 clearly shows the superior surfaces which are obtainableusing my electrolyte. For example, on occasion, surface finishes as lowas about 20 microinches may be obtained on steel machined with sodiumchloride electrolytes. However, much rougher surfaces are normallyobtained. I can machine the same steel over twice as fast with myelectrolyte solution and still consistently obtain surface finishes of2-5 microinches.

It is to be understood that although this invention has been describedin connection with certain specific examples thereof no limitation isintended thereby except as defined in the appended claims.

I claim:

1. A method for the precise electrochemical machining of a selected areaof a ferrous metal workpiece comprising the steps of: making saidworkpiece an anode in an aqueous electrolyte solution consistingessentially of at least about 100 grams per liter of at least one saltselected from the group consisting of sodium chlorate, potassiumchlorate, sodium perchlorate and potassium perchlorate; positioning acathode adjacent said selected area in said electrolyte solution;forming an electrochemical erosion-inhibiting film on the surface ofsaid workpiece; and selectively electrochemically removing a portion ofsaid film to expose said area and anodically consuming said workpiece atsaid area by establishing a sufficient potential between said workpieceand said cathode to effect an anodic current density of at least about100 amperes per square inch on said area during said machining.

2. The method of claim 1 wherein said electrolyte solution contains atleast about 350 grams per liter of sodium chlorate and the appliedpotential is sufficient to establish a current density of at least about175 amperes per square inch.

3. A method for the precise electrochemical machining of a ferrous metalselected from the group consisting of iron, ironcarbon alloys, alloysteels, stainless steels, and high-temperature alloys comprising thesteps of: making said metal an anode in an aqueous electrolyte solutionconsisting essentially of at least about 350 grams per liter of sodiumchlorate and having a pH of about 7-l0.4 for said iron-carbon alloys andalloy steels and a pH of about 8-13 for said stainless steels andhigh-temperature alloys; positioning a cathode adjacent to said metal insaid electrolyte solution; forming electrochemical erosion-inhibitingfilm on the surface of said metal; and selectively electrochemicallyremoving a portion of said film to expose a selected area of said metaland anodically consuming said metal at said area by establishing asufficient potential between said metal and said cathode to effect ananodic current density of at least about 175 amperes per square inch onsaid area during said machining.

4. The method according to claim 3 wherein the temperature of saidelectrolyte is between about F. and F.

2. The method of claim 1 wherein said electrolyte solution contains atleast about 350 grams per liter of sodium chlorate and the appliedpotential is sufficient to establish a current density of at least about175 amperes per square inch.
 3. A method for the precise electrochemicalmachining of a ferrous metal selected from the group consisting of iron,iron-carbon alloys, alloy steels, stainless steels, and high-temperaturealloys comprising the steps of: making said metal an anode in an aqueouselectrolyte solution consisting essentially of at least about 350 gramsper liter of sodium chlorate and having a pH of about 7-10.4 for saidiron-carbon alloys and alloy steels and a pH of about 8-13 for saidstainless steels and high-temperature alloys; positioning a cathodeadjacent to said metal in said electrolyte solution; formingelectrochemical erosion-inhibiting film on the surface of said metal;and selectively electrochemically removing a portion of said film toexpose a selected area of said metal and anodically consuming said metalat said area by establishing a sufficient potential between said metaland said cathode to effect an anodic current density of at least about175 amperes per square inch on said area during said machining.
 4. Themethod according to claim 3 wherein the temperature of said electrolyteis between about 100* F. and 160* F.